The present invention relates to a procedure for stabilizing an iron-air battery comprising one of several cells with a positive electrode containing at least one of the metals cobalt, nicket and silver for the reduction of the oxygen of the air during discharge and the development of oxygen during charge, an alkaline electrolyte and a negative electrode containing electrochemically active iron, whereby a sulphur-containing compound is added to the electrolyte. The invention also refers to an iron-air battery cell stabilized in this way.
The iron air battery is one of the most promising electrochemical power sources for tractionary application on large scale. The energy density is high, on the order of 100 Wh/kg, and the power density is sufficient for most applications, i.e., on the order of 20- 40 W/kg. The iron-air battery also uses cheap electrode materials, which furthermore do not constitute hazards to the environment during the steps of manufacture, use and recovery.
This power source is, however, not completely problem-free. It has certainly been found possible to make iron electrodes with a very high capacity density and good mechanical properties, for instance according to the procedure which is described in the Swedish Pat. No. 360,952. However, even these iron electrodes exhibit, as do other iron electrodes, certain problems which are connected with the electrode material as such, namely a tendency to self-discharge and the occurrence of variations in capacity, depending on the mode of operation and the discharge pattern.
These problems have been known since the pioneering work on alkaline accumulators was carried out by Thomas Alva Edison, and Waldemar Jungner. It is known that it is possible to improve the properties of the iron-air electrode by the addition of mercury oxide which was used by Edison and is described for instance in U.S. Pat. No. 692,507, or by the addition of sulphur or sulphur-containing compounds to the electrolyte or to the iron electrode as described for instance in Swedish Pat. No. 196,168 and the German Offenlegungs publication No. 2,164,208. Mercury is, however, a less useful material in this connection because of the high price of the metal and the risk that mercury vapour will be released to the atmosphere through the air electrode which could become a serious hazard to the environment. The use of sulphur or sulphur-containing compounds according to the state of the art has not been able to stabilize the iron electrode in most iron-air batteries.
The use of very high sulphur contents has so far been practiced, i.e., the Swedish Pat. No. 196,168 recommends a sulphide electrolyte containing 38 grams Na.sub.2 S. 9H.sub.2 O in 200 ml of electrolyte, which means that the sulphur contained in this electrolyte will be about 20.000 ppm (parts per million) of the weight of the electrolyte. The use of such high sulphur contents in an attempt to stabilize iron-air batteries having iron anodes and cathodes containing some of the metals nickel, cobalt and silver as supporting and catalytically active components in the electrode has the result has been that these cells become inactivated very rapidly because the function of the cathode becomes eliminated.
Air cathodes of this type contain small quantities of oxides and hydroxides of the mentioned metals which produce an electrocatalytical action in the electron charge transfer step and a catalytic action on the decomposition of peroxide produced in the cathode reaction. These active metal compounds apparently react with the sulphide additive forming inactive compounds when sulphide is added to the electrolyte according to this procedure of the state of the art. This is a very difficult problem since iron-air batteries according to the state of art, which can reduce the oxygen of the air during discharge and develop oxygen during charge, generally contain one or several of the metals mentioned above. Such an air cathode is described in Swedish Pat. No. 360,952 for example.
It is, therefore, an object of the present invention to provide a method of stabilizing an iron-air battery.
It is another object of the invention to provide a stabilized iron-air battery.